For a hydrostatic level probe on an anaerobic digestion plant, the choice is between a well-known premium make and a much cheaper alternative that has already been used on other plants. For a palm oil mill digester in Indonesia, the quotation offered both, along with differential pressure transmitters for gas flow.
The challenge: instruments for a repeat plant
The client was ordering further instruments for a digestion plant already supplied. The supplier had since used a lower-cost alternative on three further digestion projects in Indonesia.
The approach: spec, option and lead time
- Differential pressure transmitters (two): set range 0 to 20 mbar for a flow of 0 to 1,700 m³/h, 24 V DC intrinsically safe, with calibration certificate. Lead time two weeks.
- Hydrostatic level probe: 4 to 20 mA output, 316L tube of 22 mm diameter, range 0 to 0.6 bar, EPDM seal and a 10 m cable. Lead time two weeks.
- Economy option for the level probe: a hydrostatic level meter described as substantially cheaper, already used on three subsequent plants. Lead time three weeks.
- Low-range differential pressure transmitter: set range 0 to 5 mbar, 100 bar static pressure, intrinsically safe. Lead time 10 to 12 weeks, much longer than the others.
A separate enquiry from the client’s procurement team asked for temperature and pressure instruments: PT100 temperature transmitters (0 to 100°C) with thermowells, and 4 to 20 mA pressure transmitters in ranges of 0 to 6 bar and 0 to 0.3 bar, the latter with ATEX Ex ia protection.
Why it works
The pressure ranges are tiny. A range of 0 to 20 mbar or 0 to 0.3 bar matches the low pressures of a digester gas system, where a standard 0 to 6 bar transmitter would have poor resolution. Intrinsically safe (Ex ia) circuits limit the energy available to ignite gas. Offering an economy option puts the cost decision with the buyer, with proof from three other plants.
For UK context, see the HSE’s ATEX guidance.
What to consider for your own project
- Match the range to the pressure. Small ranges for low-pressure gas.
- Check hazardous area certification of every instrument.
- Look at lead times. One item took 10 to 12 weeks.
- Ask for field experience before choosing the cheaper make.
- Specify cable length for submersible probes.
For a similar pattern of substitution, see our post on replacing obsolete instrumentation and controls.
FAQ
What is a hydrostatic level probe?
A probe that measures liquid level by the pressure of the liquid above it.
What does Ex ia mean?
An intrinsically safe circuit that cannot release enough energy to ignite gas.
Why is the low-range transmitter slow?
It is a specialised item with a longer manufacturing time.